(Single-Domain VA (Vertical Alignment) Method for Liquid Crystal Panels)
First, a single-domain VA method for liquid crystal panels will be described. In FIG. 71, (a) to (d) are graphs illustrating relationships between viewing angles and relative luminance in a conventional single-domain VA method, and (e) is a diagram illustrating an alignment direction of a liquid crystal molecule 41 in the single-domain VA method.
The alignment direction of the liquid crystal molecule 41 illustrated in FIG. 71(e) will be described herein using an alignment vector at a time when a liquid crystal panel is viewed in a plan view, an end of a longitudinal axis of the liquid crystal molecule 41 closer to a first substrate is determined as an initial point 41S, and an end of the longitudinal axis closer to a second substrate is determined as a final point 41T.
FIG. 71(a) illustrates a relationship between viewing angles in directions indicated by arrows A and E in FIG. 71(e) and the relative luminance of the liquid crystal molecule 41 with various tones. FIG. 71(b) illustrates a relationship between viewing angles in directions indicated by arrows B and F and the relative luminance with the various tones. FIG. 71(c) illustrates a relationship between viewing angles in directions indicated by arrows C and G and the relative luminance with the various tones, and FIG. 71(d) illustrates a relationship between viewing angles in directions indicated by arrows D and H and the relative luminance with the various tones.
As illustrated in FIG. 71, in the single-domain VA method, changes in relative luminance with the various tones due to viewing angles are large, and there is no symmetry in up, down, left, and right directions. In the up and down directions, black reversal and white reversal occur. In particular, when black reversal occurs, a natural image looks like a negative-positive reversal image, which poses a serious problem in terms of display quality.
Viewing angle characteristics are thus problematic in the single-domain VA method for liquid crystal panels. In order to solve the problem of viewing angle characteristics, the following MVA (multi-domain vertical alignment) method and MPD (multi-pixel drive) technique were developed.
(MVA Method for Liquid Crystal Panels)
FIGS. 72(a) to (d) are graphs illustrating relationships between viewing angles and relative luminance in the conventional MVA method, and (e) is a diagram illustrating alignment directions of liquid crystal molecules in the MVA method.
FIG. 72(a) illustrates a relationship between viewing angles in directions indicated by arrows A and E in FIG. 72(e) and the relative luminance of four liquid crystal molecules 41 whose alignment directions are different from one another with various tones. FIG. 72(b) illustrates a relationship between viewing angles in directions indicated by arrows B and F and the relative luminance with the various tones. FIG. 72(c) illustrates a relationship between viewing angles in directions indicated by arrows C and G and the relative luminance with the various tones, and FIG. 72(d) illustrates a relationship between viewing angles in directions indicated by arrows D and H and the relative luminance with the various tones.
In the MVA method, a plurality of, generally four, domains whose alignment directions (directions in which the liquid crystal molecules 41 are tilted when voltage is applied) are different from one another are provided. As a result, viewing angle characteristics that had been asymmetric in the up, down, left, and right directions could be significantly improved to substantially symmetric characteristics. Especially black and white reversal phenomena in the up direction could be significantly improved, and the problem that a natural image looked like a negative-positive reversal image was substantially solved (PTL 1). As a result, liquid crystal panels employing the MVA method were widely used for TV purposes.
As described above, in the MVA method, viewing angle characteristics were significantly improved by providing a plurality of domains whose alignment directions were different from one another, and viewing angle characteristics symmetric in the up, down, left, and right directions were achieved. Black reversal was eliminated, and white reversal was significantly improved.
(MPD Technique for Liquid Crystal Panels)
After viewing angle characteristics were significantly improved by introducing the MVA method, the MPD technique was developed in order to achieve further improvements. In the MPD technique, each pixel is divided into a plurality of, generally two, subpixels to which different voltages can be applied (PTL 2 and PTL 3). More specifically, a bright subpixel that exhibits a luminance higher than a luminance achieved by a displayed intermediate tone and a dark subpixel that exhibits a luminance lower than the luminance achieved by the displayed intermediate tone are provided. As a result, changes in luminance (also called “white glow”) and changes in color when an intermediate tone is displayed can be significantly improved. This is because viewing angle characteristics (changes in luminance in oblique view) of a four-domain VA mode tend to be better with an intermediate tone of a relatively high luminance than with an intermediate tone of a relatively low luminance. Furthermore, whereas luminance tends to increase in the case of a relatively low luminance as a viewer switches from front view to oblique view, luminance tends to show an opposite tendency, that is, decrease, in the case of a relatively high luminance as a viewer switches from front view to oblique view. When an intermediate tone is displayed, therefore, viewing angle performance can be improved by combining a bright subpixel, which is bright, and a dark subpixel, which is dark, with each other and exhibiting a desired luminance as an average in a whole pixel.
FIG. 73(a) is a diagram illustrating tilt directions of liquid crystal molecules 41 without the MPD technique, and (b) and (c) are graphs illustrating relationships between viewing angles and relative luminance without the MPD technique. (d) is a diagram illustrating tilt directions of liquid crystal molecules with the MPD technique, and (e) and (f) are graphs illustrating relationships between viewing angles and relative luminance with the MPD technique.
FIG. 73(b) illustrates a relationship between viewing angles in directions indicated by arrows A and E in FIG. 73(a) and the relative luminance of the liquid crystal molecules 41 with various tones. FIG. 73(c) illustrates a relationship between viewing angles in directions indicated by arrows B and F and the relative luminance with the various tones.
FIG. 73(e) illustrates a relationship between viewing angles in directions indicated by arrows A and E in FIG. 73(d) and the relative luminance of the liquid crystal molecules 41 with the various tones. FIG. 73(f) illustrates a relationship between viewing angles in directions indicated by arrows B and F and the relative luminance with the various tones.
When the MPD technique is used as in FIGS. 73(d) to (f), changes in luminance when a viewing angle has changed are smaller and better viewing angle performance is achieved than when the MPD technique is not used as in FIGS. 73(a) to (c). In the case of a tone of 96/225, for example, changes in luminance in the A-E directions illustrated in FIG. 73(b) is 2.2 times at maximum in a front luminance ratio and 2.2 times in the B-F directions illustrated in FIG. 73(c) when the MPD technique is not used. When the MPD technique is used, on the other hand, the changes are 1.3 times at maximum in the A-E directions illustrated in FIGS. 73(e) and 1.4 times in the B-F directions illustrated in FIG. 73(f), which indicates significant improvements.
Viewing angle characteristics, which had posed a problem in the single-domain VA method, no longer posed a problem as a result of the introduction of the MVA method and the MPD technique.